# Fault Calculations - Typical Equipment Parameters

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A frequent problem in fault calculations is the obtaining of equipment parameters. While it is always preferable to use the actual parameters of the equipment, sometimes these are not available. In this instance it is necessary to resort to the use of typical parameters.

## HV Network Impedance

Impedance of HV network referred to LV side of transformer

 Psc Uo (V) Ra (mΩ) Xa (mΩ) 250 MVA 420 0.106 0.71 500 MVA 420 0.053 0.353

## Low Voltage Power Transformers

### Transformer Short Circuit Impedance

Typical values short circuit impedance, USC for different kVA ratings (HV winding < 20 kV)

 Type of Transformer transformer rating oil-immersed cast-resin 50 to 630 4% 6% 800 to 2500 6% 6%

### Transformer Impedance (400 V)

Resistance, reactance and impedance for typical transformers (HV winding <20 kV)

 transformer rated power (kVA) 50 100 160 250 315 400 500 630 800 1000 1250 1600 2000 2500 oil-immersed Usc % 4 4 4 4 4 4 4 4 6 6 6 6 6 6 Rtr mΩ 95.3 37.9 16.2 9.2 6.9 5.1 3.9 2.9 2.9 2.3 1.8 1.4 1.1 0.9 Xtr mΩ 104.1 59.5 41 26.7 21.3 16.9 13.6 10.8 12.9 10.3 8.3 6.5 5.2 4.1 Ztr mΩ 141.1 70.5 44.1 28.2 22.4 17.7 14.1 11.2 13.2 10.6 8.5 6.6 5.3 4.2 cast-resin Usc % 6 6 6 6 6 6 6 6 6 6 6 6 6 6 Rtr mΩ 33.5 18.6 10.7 8.2 6.1 4.6 3.5 2.6 1.9 1.5 1.1 0.8 0.6 Xtr mΩ 100.4 63.5 41 32.6 25.8 20.7 16.4 13 10.4 8.3 6.5 5.2 4.2 Ztr mΩ 105.8 66.2 42.4 33.6 26.5 21.2 16.8 13.3 10.6 8.4 6.6 5.3 4.2

## Synchronous Machines

### Performance Under Fault Conditions

Sub-transient emf:

Transient emf:

Synchronous emf:
Fault current:

## Typical Machine Reactances

Typical Machine Reactances (& Time Constants

 Cylindrical Rotor Turbine Generators Salient Pole Generators Salient Pole Synchronous Condensers Conventional Direct Cooled 4 Pole Multi-Pole Synchronous Direct axis Xd pu 1.6-2.0 0.8-1.0 2.0-2.3 2.1-2.4 1.3-2.1 1.3-1.5 Quadrature axis Xq pu 1.0-1.25 0.5-0.65 1.95-2.1 1.95-2.25 0.6-1.2 0.8-1.0 Transient Reactance Xd' pu 0.3-0.5 0.2-0.35 0.18-0.25 0.27-0.30 0.15-0.35 0.4-0.5 Short circuit τd' s 1.5-2.5 1.0-2.0 0.75-1.0 0.75-1.0 0.8-1.2 0.8-1.2 Open circuit τdo' s 5-10 3-7 4-8 6-9.5 4-8 3-7 Sub-transient Direct axis Xd'' pu 0.2-0.4 0.12-0.25 0.11-0.13 0.19-0.23 0.1-0.25 0.2-0.35 Quadrature axis Xq'' pu 0.25-0.6 0.15-0.45 0.11-0.13 0.19-0.23 0.14-0.35 0.2-0.35 Short circuit - direct axis τd' s 0.04-0.09 0.05-0.10 0.015-0.025 0.02-0.03 0.02-0.04 0.02-0.07 Open circuit - direct axis τd'' s 0.07-0.11 0.08-0.25 0.015-0.025 0.02-0.03 0.04-0.07 0.02-0.07 Short circuit - quadrature axis τq'' s 0.04-0.6 0.05-0.6 0.015-0.025 0.02-0.03 0.10-0.15 0.02-0.07 Open circuit - quadrature axis τq0'' s 0.1-1.2 0.2-0.9 0.015-0.025 0.02-0.03 0.3-0.7 0.1-0.2 Other Reactances Negative sequence X-ve pu 0.25-0.5 0.14-0.35 0.11-0.13 0.19-0.23 0.12-0.3 0.2-0.35 Zero sequence Xo pu 0.12-0.16 0.06-0.10 0.05-0.075 0.11-0.16 0.03-0.10 0.1-0.2

## Cables

where:

d = distance between centre
a = cross sectional area (mm)

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Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

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